A conventional computer system typically includes a hierarchy of memory devices, such as registers, multi-level caches, main memory and secondary storage, in the order from high to low. In general, memory at a higher level of the hierarchy has higher speed (e.g., lower access latency) and lower capacity than memory at a lower level of the hierarchy. The general performance at each level has increased over time.
In conventional computer systems, the main memory is typically implemented by dynamic random access memory (DRAM). One of the commonly-used DRAM-based memory devices is the Double Data Rate Synchronous Dynamic Random-Access Memory (DDR SDRAM), also referred to as DDR. The fourth generation DDR, referred to as DDR4, is one of the latest variants of DRAM that has a high bandwidth interface. DDR4 typically operates at a voltage of 1.2 V with a frequency between 800 and 1600 MHz. As DDR in general offers a large capacity and high bandwidth, it is often the memory of choice in high-performance computers, such as server computers.
Recent advance in memory technology has brought to the market many viable alternatives to DDR. These alternative memory devices may consume lower power, cost less, and/or have higher speed than DDR. Despite these alternatives, DDR usage continues to dominate the existing memory technology. One main reason for the dominance of DDR, especially for high-performance computers, is its large capacity compared to the alternatives.